Fluvial Processes and Landforms

What are fluvial processes?Exogenetic processes

• River erosion• River transportation• River deposition

Fluvial processes

River erosion

Hydraulic action• Force of running water

Ways of erosion:

Abrasion / corrasion

• Load of a river

Attrition• Rock fragments rub and

hit against each otherSolution /

corrosion• Water as a solvent

Directions of river erosion:Headward erosion

Vertical erosion

•Lateral erosion

Compare the characteristics of the two valleys shown in Figure 14.5. Narrow V-shaped Broad V-

shaped(a) (

b)

(a) (

b)

Account for their differences in terms of the main erosional process carried out by the river.

• Vertical erosion (riverbed)

• Deepening of a river channel

• Lateral erosion (outside banks)

• Widening of a river channel

River transportationFloating small rock fragments carried downstream

Smaller rock fragments bounced downstream

Dissolved materials carried downstream

Large rock fragments dragged downstream

Nature of load Vs. Ways of transportation

• Soluble minerals are removed by solution• For the insoluble load, the largest one is pushed

along the river bed by traction, the smaller one is carried by saltation, while the finest one is suspended in the moving water.

Smallest----------------------------------------------largestSoluble ----------------------------------------------

insoluble

Solution Suspension Saltation Traction

Dissolved loadSuspended load Bed load

River deposition• Dropping of a river’s load• Occurs when load > river’s

carrying capacity

Conditions favourable for river deposition:

• Decrease in the velocity

• Decrease in the volume• Increase in the load

Volume of water• In times with rainfall

• In times with drought or low rainfall (over a long period of time)

Other conditions for a lower volume• river flows into arid areas• river flows across areas of permeable rocks• flood water subsides

Where deposition takes place:

• At any point of a river• Inside banks of bends• Floor of the river valley

Deposition

-----------------------------smallest particlePebbles small stones sandsilt clay

Sorting of fluvial deposits The smaller the particle size, the further it is carried

large Energy small

Deposition takes place …At any point of a river with lower energy• Inside banks of winding channel

• Floor of the river valley

B

outer

inner

outer

inner

depositionerosion

BA

Inner bank

Outer bank

Main fluvial process

Outer bank

Inner bank

erosion

deposition

Study the Figure 1 Describe the relationship between

erosion and deposition.

Erosion and deposition are balanced throughout a river.

Deposition reduces load and encourage erosion to take place.

Erosion provides load for deposition to take place.

Back!

2 (a) How the amount of load affects the level

of energy of a river?

When the river has too little load, its energy increases and vice versa.

2 (b) What are the other factors affecting the energy of a river?

Gravity Channel friction

Channel discharge

River velocity

Roughness

Shape

Roughness

Slope

Back!

3 Explain why erosion and deposition occur?

When the river has a lot of energy, erosion takes place. When the river has little energy, deposition resumes. Back!